They're about to get loaded with a bunch of furniture from all the biotech firms downsizing

Don't know about Boston per se but I've bought some file cabinets from a place in downtown Worcester in the past. Might be Northeast Office Solutions.

I went to Brooks Bargain in Wilmington, MA, to get a used Aeron chair for about $500 if I remember right. They had a ton of different kinds of chairs to try on site. I went there torn between Aeron or SteelCase.

I’ve heard good things about Granite State Office Furniture too, if you don’t mind hoofing it up to Manchester NH.

There's always the MIT Furniture Exchange! [Note to non-Boston folks: this is a joke.]